The state of the art of turbine nozzle design has reached a plateau regarding the manufacturing of internal geometries having complex shapes. Internal geometries for turbine nozzles which have compound internal curves are not readily machinable by conventional machining processes at a reasonable expense which permits implementation.
EDM (electrical discharge machining) and ECM (electrical chemical machining) are now in use for the manufacture of turbine nozzles. EDM has a high cost and requires post machining processes to achieve smooth finishes. The EDM process creates a "recast layer" which requires grinding and polishing for proper surface finish. Moreover, EDM, as well as all available machining processes to date, are limited in producing a repeatable internal geometry. EDM is also limited as to the types of internal geometry and depth of cut.
It is desired for the internal geometries of turbine nozzles to have a smooth mirror-like finish to minimize flow resistance on gases flowing through the nozzle. Accordingly, for turbine nozzles having complex internal geometries there is a need for a process which permits smooth mirror-like finishes to be obtained on the surfaces of the internal geometries without requiring machining or other processing steps.
A process known as investment casting is in wide use in lower technological applications. With investment casting, the mold in which a product is to be cast is produced by surrounding an expendable pattern with a refractory material that sets at room temperature. The pattern, which conventionally is in the form of wax or plastic material, is then melted or burned out of the mold, leaving the mold cavity that receives the hot metal from which the final product is formed. After the cast metal is cooled, the mold is broken away from the product. For example, see U.S. Pat. No. 4,108,931. However, the process of investment casting is not usable for making turbine nozzles with complex internal geometries which have a mirror finish without machining or other post-casting operations to obtain the desired refractory finish.